Developing an HIV-1 vaccine that elicits a potent neutralizing antibody response is a global health priority, but this response is not associated with protection in natural infections. Identifying mechanisms to improve humoral immunity against HIV-1 may therefore critically guide vaccine development efforts. In contrast to HIV- 1 infection in humans or SIV infection in macaques, resistant strains of mice infected with Friend retrovirus (FV) develop neutralizing antibody responses that are critical for recovery. Furthermore, vaccine protection against FV is dependent on neutralizing antibody responses. Interestingly, the FV-specific neutralizing antibody response is significantly influenced by a key host gene, Rfv3. We recently identified Rfv3 as Apobec3, a deoxycytidine deaminase with broad activity against retroviruses, including HIV-1 (Santiago ML et al. 2008. Science 321:1343-46). Apobec3 restricts retroviruses in the next target cell, facilitating the release of fusion- competent, non-infectious virions that may function as natural B-cell immunogens in vivo. Unraveling the fascinating link between Apobec3 and the humoral arm of the immune response may therefore have important implications for HIV-1 vaccine development. Our overall objective is to probe the Apobec3-neutralizing antibody link in two pathogenic retrovirus systems. In Specific Aim 1, we propose to determine the underlying mechanism for the Apobec3/Rfv3 phenotype in the FV murine model. This would involve monitoring virus- specific B cell development, interrogating the role of Type I Interferon regulation of Apobec3 in vivo, and assessing the molecular properties that distinguish a protective versus a non-protective antibody response. Notably, primate lentiviruses antagonize the simian and human homologues of Apobec3 through the action of Vif. Thus, attenuating Vif function may rescue Apobec3 and improve neutralizing antibody responses. In Specific Aim 2, we propose to determine the impact of Vif attenuation on humoral immunity against SIV infection. This would involve comparing Apobec3 function, viral evolution, B cell phenotypes and neutralizing antibody development in rhesus macaques infected with wild-type, Vif-attenuated and Nef-deleted SIVmac239. SIVmac239 rarely elicits a strong neutralizing antibody response and triggers rapid and extensive CD4+ T cell depletion in the gastrointestinal tract, likely weakening mucosal immune responses. By analyzing sequential fecal and urine samples, we will test whether enabling Apobec3 function will improve virus-specific mucosal IgA responses. If the Apobec3/Rfv3 phenotype in mice extends to primate lentivirus infections, the molecular properties of induced systemic and mucosal virus-specific antibodies may yield crucial and innovative insights for HIV-1 vaccine design. Detailed insights on the interplay between Apobec3-mediated innate immunity and retrovirus-specific neutralizing antibody development may critically guide the construction and evaluation of HIV vaccines.